In general, a rotor of a steam turbine is provided at a tip end thereof with a stub shaft on which a main oil pump, a safety governor, and a main governor for controlling a rotational speed of the turbine are mounted. The main oil pump serves to drive a valve for controlling an amount of steam flowing into the turbine as well as to circulate a lubricant through a rotary portion of the turbine. Generally, the main oil pump is disposed at a position most adjacent to the turbine. In this type of turbine, if the stub shaft is broken between the safety governor and the main oil pump the main governor, provided at the end of the stub shaft, reacts as if the rotational speed of the turbine is reduced, and then issues an opening command to a control valve. By virtue of the issuance of an opening command to the control valve with a broken stub shaft, the possibility exists that a greater amount of steam than necessary is allowed to flow into the turbine so as to cause the turbine to operate at an excessive rotational speed, thereby bringing about a serious accident such as, for example, a destruction of the turbine blades.
In order to prevent the occurrence of an accident of the aforementioned type, in, for example, Japanese Patent Unexamined Publication No. 57-113912, an apparatus is provided for detecting a breakage of the stub shaft, with the apparatus including an electromagnetic pick-up provided at the end of the stub shaft. When a signal issued from the electromagnetic pick-up indicates a reduction in the rotational speed and a 10 signal from a device for detecting a main oil pump discharge pressure indicates the rated discharge pressure, the turbine is immediately tripped.
In the above noted conventional apparatus, since the main oil pump discharge pressure and the rated oil pressure are compared with each other, it is possible to detect the breakage of the stub shaft during operation at rotational speed approximately corresponding to the rated rotation speed of the turbine. However, when the turbine is operated at rotational speeds less than the rated rotational speed when, for example, the rotational speed is increased and decreased, it is impossible to detect the breakage of the stub shaft.